1. Field of the Invention
In retrieving or salvaging objects from the bottom of oceans or lakes, a diver has need to send things to the surface without himself surfacing. This invention is in the field of devices the diver can carry in sufficient numbers to salvage several objects in a single dive. Such devices allow the diver to make better use of his diving time and reduce the number of decisions--to salvage or not to salvage--required of the diver.
2. Description of Prior Art
The common object-lifting device used by divers is a fixed-volume bag, which is inverted, attached to the object to be lifted, and filled with enough gas to lift the object. As the device rises above the ocean floor pressure decreases rapidly. The volume of the gas trapped inside it increases, and as the lifting force is proportional to that volume, the lifting force rises rapidly. Thus, when the loaded device reaches the surface, it is moving upwardly quite rapidly. Frequently the bag pops up above the surface. If on falling back it lands on its side or out-of-square, the bag may lose its gas and its buoyancy. Then the device and object sink again, perhaps unnoticed by the diver.
Several improvements in lift devices overcome this basic problem. A bleed valve on top of the bag may be manipulated to control the rate of ascent, or a dump cord attached to the bag may rock it to allow excess gas to escape. Both these require that the diver ascend with the device or manipulate it by lines from the ocean floor during the ascent. U.S. Pat. Nos. 2,451,002, 2,635,574, 3,659,299, 3,208,475, and 3,940,814 cover a variety of means to control dumping of excess gas in an ascending lift bag. These means all have internal moving parts and seals which may be subject to handling damage, fouling, or corrosion in the service environment. Miller (U.S. Pat. No. 3,631,551) uses the simpler expedient of a movable hole in the sidewall of a cannister to allow the diver to control the volume of buoyancy gas. The means for achieving movability of the hole--a zipper which closes both ends--is also subject to handling damage or fouling.